Research On Computer-Generated Moiré Profilometry

Structured light three-dimensional measurement method is widely used in industrial detection,artificial intelligence,manufacturing,medical diagnosis and other fields due to its advantages,such as non-contact,high measuring speed,full field measurement,simple operation,simplicity and automation,and so on.With the development of modern science and technology,the requirements of measurement accuracy in all walks of life are also constantly increasing,which makes various of three-dimensional measurement technology based on structured light projection develop and improve constantly.In this thesis,Moiré fringes generation technology and phase-shifting technology are combined,and Moiré fringes with different shifted phases can be obtained by computer superposition,so as to realize the phase calculation of the measured object by only one deformed pattern and reconstruct its 3D surface.The method studied in this thesis is still based on the phase to height mapping method.The traditional Moiréfringe is mainly generated by multiplying the pass rate of two gratings,and the frineg contrast is low.When the Moiré fringe is generated by computer through image processing,the fringe can be processed first and then superimposed to improve the proportion of Moiré component in the superimprosed result.In this thesis,a computergenerated moire profilometry is proposed to achieve high precision real-time 3D measurement,and the accuracy improvement of the method is studied.These method need to use two-frame fringe projection when measuring.In order to ensure their realtime measurement ability,the time-division multiplexing projection and control of the camera synchronous acquisition are used to apply these algorithms in the real-time 3D measurement scene.The main contents of this thesis include the following aspects:1.A computer-generated Moiré profilometry is studied and proposed(CGMP).The three-dimensional shape of the measured object is reconstructed only by single frame grating projection and single frame deformed pattern acquisition.The background light component of the deformed pattern can be eliminated by taking Fourier transform of the deformed pattern and filtering out its zero frequency component.Then,two frames of Moiré fringes are generated by multiplying the deformed pattern of the measured object and the pre-stored deformed pattern of the reference plane.Moiré fringes contain the phase information caused by the object,and the phase difference between them is π/2 by which the phase information caused by the measured object can be calculated directly,so as to reconstruct the threedimensional surface of the measured object.The single frame characteristic of this method makes it can be effectively applied to real-time measurement scene.By projecting the digital sinusoidal grating to the surface of the measured object,and collecting the motion video of the object,the three-dimensional shape of the object in each state and can be reconstructed via the procession of the fringe pattern at each moment.At the same time,an algorithm to determine the size of the filter window is proposed to realize the automatic selection of the filter window without human intervention.The feasibility and effectiveness of the method proposed in this chapter are verified by computer simulation and physical measuring experiments.Compared with the FTP method with the same single frame characteristics,the measurement accuracy of computer-generated Moiré profilometry is higher.The measurement of real-time moving objects also proves the feasibility of this method in real-time measurement.2.An improved computer-generated Moiré profilometry is studied and proposed(HCGMP).In order to improve the accuracy of computer-generated Moiréprofilometry,a extra digital sinusoidal grating with π phase difference from the original grating is introduced into the projected digital grating.In this case,two frames of deformed patterns can be captured,and the phase difference between the two frames is π,the background light component can be accurately eliminated just by subtracting two frames of fringe.Experimental results show that this method can not only effectively eliminate the background light component,but also suppress the highfrequency noise and improve the property of the deformed pattern.Then the processed deformed patterns of the measured object and the reference plane are multiplied to generate Moiré fringes,so as to calculate the phase information caused by the measured object.This method using two frames grating in projection when measuring,so that the single frame characteristic is broken.Therefore,a method of time division multiplexing(TDM)cyclic projection of two frames digital sinusoidal grating is proposed,and the synchronous signal is used to control the camera to capture the corresponding deformedpattern synchronously,so as to realize the application of improved computer-generated Moiré profilometry in the field of real-time measurement.The feasibility of this method is verified by both simulation and physical measuring experiments.At the same time,compared with the computer-generatedd Moiré profilometry and π phase-shifting FTP,this method has higher measurement accuracy.3.A computer-generated Moiré profilometry based on additive superposition is proposed(ACGMP).It uses the same method to eliminate the background light of the deformed fringes of the measured object as the improved computer-generated Moiréprofilometry,but the superposition method is changed from multiplication to addition.A new group of Moiré fringes is generated by adding the deformed pattern of the measured object with background light component eliminated and the deformed patterns of the reference plane.The positive first-order spectrum of the new generated fringes is filtered,and then multiplied with their complex conjugation to produce the Moiré fringes.The Moiré fringes contain the phase information caused by the measured object,and have the π phase difference between them.Then,the phase information caused the measured object can be calculated directly by Moiré fringes,and finally the three-dimensional shape of the measured object can be reconstructed.The background light of the deformed pattern can not be eliminated completely,there will be residual background light components left in the processed deformed fringes.According to the properties of the trigonometric function,the residual background light will remain in the zero-frequency component of the new generated fringe when it is superposed with the deformed pattern of reference plane by multiplicaiton,which will lead to the aliasing of this interference information and Moiré fringe,thus affecting the measurement results.By addition superposition,the zero-frequency component of the original fringe is remain in the zero-frequency component of the new generated fringe,but the effective Moiré component is the fundamental frequency component of the new fringe,so the residual background light component can be separated out effectively.Simulation experiments and physical measuring experiments verify the feasibility and effectiveness of the method.4.A frequency-carrier computer-generated Moiré profilometry is studied and proposed(CCGMP).Moiré fringes generated by superposition of deformed fringe of the measured object and deformed fringes of reference plane always introduce the interference component of reference plane into Moiré fringes.In order to improve the quality of Moiré fringes to the greatest extent,the sinusoidal grating directly generated by computer is introduced as the reference grating,which is superimposed with the processed deformed pattern of the measured object and the processed deformed pattern of the reference plane respectively.Two groups of Moiré fringes modulated by carrier frequency can be generated.This method can avoid the noise of the reference plane grating effectively,and improve the quality of the Moiré fringes.At the same time,the influence of the frequency of the simulated grating generated by computer on the measurement results is analyzed,and the optimal frequency is determined to be 1/4 line per pixel.The feasibility and effectiveness of carrier frequency computed Moire profilometry are verified by simulation experiments and physical measuring experiments.